For links that are several thousand kilometers long, systems have already been proposed comprising a cascade of erbium-doped optical fiber optical amplifiers.
The article "Demonstration of transmission at 2.4 Gbit/s over 12,000 km" by L. F. Mollenauer, B. M. Nyman, M. J. Neubelt, G. Raybon, and S. G. Evangelides, published in Electronics Letters, Jan. 17, 1991, Vol. 27, No. 2, describes a 12,000 km link using signals at a wavelength of 1531.9 nm.
The article entitled "Non-regenerative optical transmission experiments using 12 Er. doped fiber amplifiers" by Noboru Edegava. Yoshihiro Yoshida. Hidenori Taga. Shu Yamamoto. Kiyofumi Mochizuki, and Hiroharu Wakabayashi, published in ECOC' 89, post deadline paper PDA-8, Gothenburg 1989, describes a 902 km link using signals at a wavelength of 1536 nm.
The article entitled "Demonstration of optical pulse propagation over 10,000 km of fiber using recirculating loop" by D. J. Malyon, T. Widdowson, E. G. Bryant, S. F. Carter, J. V. Wright, and W. A. Stalard, published in Electronics Letters, Jan. 17, 1991, Vol. 27, No. 2, relates to a 10,000 km link using signals at 1560 nm.
All of the above proposed systems include optical fiber amplifiers whose silica cores are doped either with aluminum and erbium, or with aluminum, germanium, and erbium. There appears to be no need to interpose optical fibers between the amplifiers in such systems.
In contrast, if optical fibers of the above type are used for transmitting signals at wavelengths of the order of 1550 nm, it is essential to put optical fibers between the amplifiers in order to limit noise. A system of this kind is described in the article "An over 2200 km coherent transmission experiment at 2.5 Gbit/s using erbium-doped fiber amplifiers" by Shigeru Saito, Takamasa Imai, Toshihiko Sugie, Norio Ohkawa, Yasutaka Ichihashi, and Takeski Ito, published in OFC' 90, San Francisco, PD2-1.
An object of the present invention is to implement a system suitable for transmitting signals at 1550 nm, that does not require intermediate optical fibers that are too difficult to match, and that can be used on very long distance links, e.g. transatlantic links.